The effect of triiodothyronine administration on phosphokinase activity in cytosol and nuclei of hypothyroid rat liver will be studied. We will determine the effect of dosage and duration of exposure to T3 and quantitate total phosphokinase using different substrates and separate fractions by columns. Uptake of receptor by nuclei derived from animals with lowered receptor capacity will be stuied in vitro. Turnover of nuclear T3 receptor will be stuied in animals treated with protein in RNA synthesis blocking compounds. The nature of protein synthesized in response to T3 administration to hypothyroid rats will be studied by recovery of polysomal bound mRNA which will be translated in a reticulocyte lysate system. After bidimensional electrophoresis the proteins will be detected by radioautography.